Scientific Theory

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Asteroids and dinosaurs: Unexpected WZLVWVDQGDQXQQLVKHGVWRU\Plate tectonics might seem like a routine topic from a 7th grade textbook, but in the 1970s, plate tectonics was cutting-edge science. The theory had only gained widespread acceptance over the previous ten years and subsequently attracted scads of scientists looking to open up new intellectual frontiers. Walter Alvarez was one of them, but his research into plate tectonics was destined to be sidelined. An intriguing observation would eventually lead him, his collaborators, and the rest of science on an intellectual journey across geology, chemistry, paleontology, and atmospheric scienceto-wards solving one of the great mysteries in Earths his-tory: What happened to the dinosaurs (Fig. 1)?

This case study highlights these aspects of the nature of science:

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)URPSODWHWHFWRQLFVWRSDOHRQWRORJ\One of the key pieces of evidence supporting plate tec-tonic theory was the discovery that rocks on the sea-floor record ancient reversals of the Earths magnetic lELDASROCKSAREFORMEDWHEREPLATESAREMOVINGAWAYfrom one another, they record the current direction of THE%ARTHSMAGNETIClELDWHICHmIPmOPSIRREGULARLYover very long periods of time (Fig. 2). In these flip-mOPSvTHEPOLARITYOF THEMAGNETIClELDCHANGES SOthat a compass needle might point south for 200,000 years and then point north for the next 600,000 years. Walter Alvarez, an American geologist, and his collab-ORATORSWERE LOOKING FOR INDEPENDENT VERIlCATIONOFthe timing of these magnetic flip-flops in the sedimen-tary rocks of the Italian Apennine mountains. Around 65 million years ago, those sediments lay undisturbed at the bottom of the ocean and also recorded reversals of THEMAGNETIClELDASSEDIMENTSlLTEREDDOWNANDWERESLOWLYCOMPRESSEDOVERTIME&IG

As Alvarez clambered up and down the Apennines, collecting samples for magnetic analysis, he regularly con-fronted a distinct sequence of rock layers marking the 65 million year old boundary between the Cretaceous and Tertiary periodsthe KT boundary (from Kreidezeit, the German word for Cretaceous). This boundary was made up of a lower layer of sedimentary rock rich with a wide variety of marine fossils, a centimeter-thick layer

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False starts and a new leadAt the time, most paleontologists viewed the dinosaur extinction as a gradual EVENTCAPPEDBYTHElNALEXTINCTIONSAT THEENDOF THE#RETACEOUS4O!LVAREZhowever, the KT boundary certainly looked catastrophic and suddenbut the timing of the event was still an open question: was the KT transition (represented by the clay layer in the stratigraphy) gradual or sudden? To answer that question, he needed to know how long it had taken to deposit the clay layerbut how could he time an event that happened 65 million years ago? Walter Alvarez discussed the question with his father, the physicist Luis Alvarez (Fig. 4), who suggested using beryllium-10, which is laid down at a constant rate in sedimentary rocks and then radioactively decays. Perhaps beryllium could serve as a timer.

Their idea was to recruit Richard Muller, another physicist, to help measure the amount of beryllium-10 in the clay layer, correct for how much the beryllium would HAVEDECAYEDSINCETHENANDTHENREASONBACKWARDSTOlGUREOUTHOWMANYYEARS

would have had to pass for that much beryllium to be deposited. How-ever, before they could take the beryllium measurements, they learned that the published decay rate for the isotope was wrong. Calculations based on the new numbers revealed that the planned analysis would not work. For the amounts of beryllium that they could detect, the timer in the 65 million year old clay layer would have already run outall of the beryllium would have decayed away.

The beryllium investigation turned out to be a dead end, but Luis Alvarez soon came up with a replacement: iridium. Iridium is incredibly rare in the Earths crust but is more prevalent in meteorites and meteorite dust. They reasoned that since meteorite dust and hence, iridium, rain down upon Earth at a fairly constant rate, the amount of iridium in the clay would indicate how long it took for the layer to be deposited. An observa-

of claystone devoid of all fossils, and an upper layer of sedimentary rock containing a much reduced variety of marine fossils (Fig. 3).

Why the sudden reduction in marine fossils? What had caused this apparent extinction, which seemed to occur so suddenly in the fossil record, and was it re-lated to the simultaneous extinction of dinosaurs on land? Alvarez was curious and recognized that answer-INGSUCHADIFlCULTQUESTIONWOULDGARNERTHERESPECTANDATTENTIONOFTHESCIENTIlCCOMMUNITY

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tion of more concentrated iridium (around one iridium atom per ten billion other particles) would have implied slower deposition, and less iridium (an undetectably small amount) would have implied rapid deposition and a sudden KT transition, as shown in Figure 5.

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7KHSORWWKLFNHQVThe results of the iridium analysis were quite clear and completely surprising. The team (which also included chemists Helen Michel and Frank Asaro; Fig. 6) found three parts iridium per billionmore than 30 times what they had expected based on either of their hypotheses, and much, much more than contained in other stratigraphic layers (Fig. 7)! Clearly something unusual was going on at the time this clay layer was depositedbut what would have caused such a spike in iridium? The team BEGANCALLINGTHEIRlNDINGhTHEIRIDIUMANOMALYvBECAUSEITWASSODIFFERENTFROMwhat had been seen anywhere else.

.OW!LVAREZANDHISTEAMHADEVENMOREQUESTIONS"UTlRSTTHEYNEEDEDTOKNOWhow widespread this iridium anomaly was. Was it a local blipthe signal of a small-scale disaster restricted to a small part of the ancient seaflooror was the iridium spike found globally, indicating widespread catastrophe?

Alvarez began digging through published geological studies to identify a different site that also exposed the KT boundary. He eventually found one in Denmark and asked a colleague to perform the iridium test (Fig. 4HERESULTSCONlRMEDTHEIMPORTANCEOFTHEIRIDIUMANOMALYWHATEVERHADHAPPENEDATTHEENDOFTHECretaceous had been broad in scale.

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$QRWKHUIDOVHVWDUWAlvarez had embarked on the iridium analysis to re-solve the issue of the speed of the KT clay deposition, but the results sidetracked him once again, pointing to a new and even more compelling question: what caused the sky-high iridium levels at the KT boundary? The observation of high global iridium levels happened to support an existing hypothesis.

Almost ten years before the iridium discovery, physi-cist Wallace Tucker and paleontologist Dale Russell Fig. 9.7KHLULGLXPREVHUYDWLRQVXSSRUWVWKHVXSHUQRYDK\SRWKHVLV


had proposed that a supernova (and the accompanying radiation) at the end of the Cretaceous had caused the EXTINCTIONOFDINOSAURS3UPERNOVASTHROWOFFHEAVYELEMENTSLIKEIRIDIUMSOTHEHYPOTHESISSEEMEDTOlTperfectly with the teams discovery (Fig. 9). In this case, an observation made in one context (the timing of the KT transition) ended up supporting a hypothesis that had not initially been on the researchers radar screen at all (that the dinosaur extinction was triggered by a supernova).

To further test the supernova hypothesis, the team reasoned out what other lines of evidence might be relevant. Luis Alvarez realized that if a supernova had actually occurred, it would have also released pluto-nium-244, which would have accumulated alongside the iridium at the KT boundary.

Excited about the possibility of the supernova discov-ery (strong evidence that the dinosaurs had been killed off by an imploding star would have made worldwide HEADLINESTHETEAMDECIDEDTOPERFORMTHEDIFlCULTplutonium tests. When Helen Michel and Frank Asaro came back with the test results, they were elated to have discovered the telltale plutonium! But double-checking their results by replicating the analysis led to disap-POINTMENTTHEIRlRSTSAMPLEHADBEENCONTAMINATEDBYANEXPERIMENTGOINGONINANEARBYLABTHEREWASno plutonium in the sample at all, contradicting the supernova hypothesis (Fig. 10).

7KUHHREVHUYDWLRQVRQHK\SRWKHVLVThe KT boundary layer contained plenty of iridium but no plutonium-244. Furthermore, the boundary marked what seemed to be a major extinction event for marine and terrestrial life, including the dinosaurs.

7HAT HYPOTHESIS WOULD lT ALL THOSE DISPARATE OB-servations and tie them together so that they made sense? The team came up with the idea of an asteroid impactwhich would explain the iridium (since as-teroids contain much more iridium than the Earths crust) and the lack of plutoniumbut which also led them to a new question: how could an asteroid im-pact have caused the dinosaur extinction (Fig. 11)?

Once again, Luis Alvarez came to the rescue with some calculations and an elaborated hypothesis. Talks with his colleagues led him to focus on the dust that would have been thrown into the atmosphere by a huge aster-

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oid impact. He hypothesized that a huge asteroid had struck Earth at the end of the Cretaceous and had blown millions of tons of dust into the atmosphere. According to his calculations, this amount of dust would have blotted out the sun around the world, stopping photosynthesis and plant growth and hence, causing the global COLLAPSEOFFOODWEBS&IG4HISELABORATEDVERSIONOFTHEHYPOTHESISDIDINDEEDSEEMTOlTWITHALLTHREEof the lines of evidence available so far: lack of plutonium, high iridium levels, and a major extinction event.

$VWRUPIURQWMeanwhile, word of the iridium spike at the KT BOUNDARYIN)TALYAND$ENMARKHADSPREAD3CIENTISTSaround the world had begun to try to replicate this discovery at other KT localities and had succeeded: MANY INDEPENDENT SCIENTIlC TEAMS CONlRMED THATwhatever event had led to the iridium anomaly had been global in scale (Fig. 13).

In 1980, amidst this excitement, Alvarezs team pub-lished their hypothesis linking the iridium anomaly and the dinosaur extinction in the journal Science and IGNITEDAlRESTORMOFDEBATEANDEXPLORATION)NTHENEXTTENYEARSMORETHANSCIENTIlCPAPERSWOULDBEPUBLISHEDONTHETOPIC3CIENTISTSINTHElELDSOFpaleontology, geology, chemistry, astronomy, and physics joined the fray, bringing new evidence and new ideas to the table.

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!LVAREZSTEAMHYPOTHESIZEDASPECIlCCAUSEFORAONETIMEHISTORICALEVENTTHATNOONEWASAROUNDTODIRECTLYobserve. You might think that this would make the hypothesis impossible to test or that relevant evidence WOULDBEHARDTOCOMEBY&ARFROMIT)NFACTTHESCIENTIlCCOMMUNITYPICKEDUPTHEIDEAANDRANWITHITexploring many other lines of evidence, all relevant to the asteroid hypothesis.

Extinctions: If an asteroid impact had actually caused a global ecological disaster, it would have led to the sudden extinction of many different groups. Thus, if the ASTEROIDHYPOTHESISWERECORRECTWEWOULDEXPECTTOlNDMANYEXTINCTIONSINTHEfossil record that line up exactly with the KT boundary, and fewer that occurred in the millions of years leading up to the end of the Cretaceous (Fig. 14).

Impact debris: If a huge asteroid had struck Earth at the end of the Cretaceous, it would have flung off particles from the impact site. Thus, if the asteroid hypoth-ESISWERECORRECTWEWOULDEXPECTTOlNDPARTICLESFROMTHEIMPACTSITEINTHE+4boundary layer.

Glass: If a huge asteroid had struck Earth at the end of the Cretaceous, it would have generated a lot of heat, melting rock into glass, and flinging glass particles away from the impact site. Thus, if the asteroid hypothesis were correct, we WOULDEXPECTTOlNDGLASSFROMTHEIMPACTATTHE+4BOUNDARY

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Shockwaves: If a huge asteroid had struck Earth at the end of the Cretaceous, it would have generated powerful shockwaves. Thus, if the asteroid hypothesis is correct, we WOULDEXPECTTOlNDEVIDENCEOFTHESESHOCKWAVESLIKETELLTALEGRAINSOFQUARTZWITHdeformations caused by the shock; Fig. 15) at the KT boundary.

Tsunami debris: If a huge asteroid had struck one of Earths oceans at the end of the Cretaceous, it would have caused tsunamis, which would have scraped up sediments from the bottom of the ocean and deposited them elsewhere (Fig. 16). Thus, if the ASTEROIDHYPOTHESISWERECORRECTWEWOULDEXPECTTOlNDDEBRISBEDSFROMTSUNAMISat the KT boundary.

Crater: If a huge asteroid had struck Earth at the end of the Cretaceous, it would have left behind a huge crater (Fig. 17). Thus, if the asteroid hypothesis were correct (and assuming that the crater was not subsequently destroyed BYTECTONICACTIONWEWOULDEXPECTTOlNDAGIGANTICCRATERSOMEWHEREon Earth dating to the end of the Cretaceous.

The evidence relevant to each of these expectations is complex (each is a lesson in the nature of science on its own!) and involved the work of scientists all around the world. The upshot of all that work, discussion, and scrutiny was that most lines of evidence seemed to be consistent with the asteroid hypothesis. The KT bound-ary is marked by impact debris, bits of glass, shocked quartz, tsunami debrisand of course, the crater.

4HEHUNDREDMILEWIDE#HICXULUBCRATERISBURIEDOFFTHE9UCATAN0ENINSULA&IG3HORTLYAFTER!LVAREZSTEAMPUBLISHEDTHEIRASTEROIDHYPOTHESISINA-EXICANOILCOMPANYHADIDENTIlED#HICXULUBASTHESITEof a massive asteroid impact. However, since the discovery was made in the context of oil exploration, it was NOTWIDELYPUBLICIZEDINTHESCIENTIlCLITERATURE)TWASNTUNTILTHATGEOLOGISTSCONNECTEDTHERELEVANTobservations (e.g., quirks in the pull of gravity near Chicxulub) with the asteroid hypothesis.

Chicxulub might seem to be the smoking gun of the dinosaur extinction (as it has sometimes been called)but in fact, it is far from the last word on the asteroid hypothesis

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THATTIME3OMEOBSERVATIONSPOINTTOADDITIONALEXPLANATIONS&URTHERRESEARCHMUCHOFITSPURREDBYTHEAS-teroid hypothesis) has revealed the end of the Cretaceous to be a chaotic time on Earth, even ignoring the issue of a massive asteroid collision. Volcanic activity peaked, producing lava flows that now cover about 200,000 square miles of India; major climate change was underway with general cooling punctuated by at least one intense period of global warming; sea level dropped and continents shifted with tectonic movements. With all this change going on, ecosystems were surely disrupted. These factors could certainly have played a role in triggering the mass extinctionbut did they?

In short, the evidence points to several potential cul-prits for the mass extinction (Fig. 19). Which is the true cause? Well, perhaps they all are.

Just as the extinction of an endangered species today may be traced to many contributing factors (global warming, habitat destruction, an invasive predator, etc.), the KT mass extinction may have been triggered by several different agents (e.g., volcanism and an as-teroid impact, with a bit of climate change thrown into the mix). If this is indeed the case and multiple causes were in play, teasing them apart will require a more integrative approach, exploring the relationships between abiotic factors (like asteroid impacts and sea level change) and extinction: which groups survived the mass extinction and which did not? Birds, for example, survived the extinction, but all other dinosaurs went extinct. What does this tell us about the cause of the extinction? Are there different patterns of extinction in different ecosystems or different parts of the world? Do these differences point to separate causal mechanisms?

0RUHNQRZOHGJHPRUHTXHVWLRQV!TlRSTGLANCETHISSNAPSHOTOFSCIENCEMIGHTSEEMTOHAVEBACKTRACKED&IRSTTHEstory is full of false starts and abandoned goals: Alvarezs work on plate tectonics was sidetracked by his intriguing observations of the KT boundary. Then his work on the timing of the KT transition was sidetracked by the iridium intrigue. The supernova hypothesis was abandoned when critical evidence failed to materialize. And now, scientists are wondering if the asteroid hypothesis can really explain the whole mass extinction. Our questions regarding the KT extinction have multi-plied since this investigation began.

All that is true; however, we also have more knowledge about events at the end of the Cretaceous than we did before Walter Alvarez (Fig. 20) began poking around in the Apennines. We know that a massive asteroid struck Earth, probably near the Yucatan Peninsula. We know that no nearby supernova rained plutonium down on Earth. We know more about the fossil record surrounding the KT. We have a more detailed understanding of the climatic and geologic changes leading up to the end of the Cretaceous. In a sense, we have so many more questions simply because we know so much more about what to ask, and this ISAFUNDAMENTALPARTOFTHESCIENTIlCENTERPRISE3CIENCEISBOTHCUMULATIVEANDCONTINUING%ACHQUESTIONthat we answer adds to our overall understanding of the natural world, but the light that is shed by that new knowledge highlights many more areas still in shadow.

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:DQWWROHDUQPRUH"&KHFNRXWWKHVHUHIHUHQFHVPopular and historical accounts:

Alvarez, W. 1997. T. rex and the crater of doom. Princeton, NJ: Princeton University Press.

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A few scientific articles:

Alvarez, L.W., W. Alvarez, F. Asaro, and H.V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Ter-tiary extinction. Science 208:10951108.

(ILDEBRAND!2'40ENlELD$!+RING-0ILKINGTON:!#AMARGO3"*ACOBSONAND76Boynton. 1991. Chicxulub crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology 19:867871.

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Scientific Theory

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